Process for treating underground formations

ABSTRACT

A process for disrupting filter cake in an underground formation, which includes (i) incorporating into a treatment fluid a solid polymer capable of being converted by hydrolysis into one or more organic acids; (ii) introducing the treatment fluid into the underground formation; and (iii) allowing the solid polymer to hydrolyze in the presence of water to produce organic acid such that acid soluble material within the filter cake or adjacent formation is dissolved.

The present invention relates to the optimization and enhancement of theproduction of oil, gas or water from wells drilled into undergroundformations. More particularly the invention relates to the disruption,especially the degradation and removal, of filter cake in undergroundformations, including the removal of filter cake in gravel pack andother sand control completions.

The effective removal of formation damage, especially near wellboredamage such as filter cake, can significantly increase the productionrate of hydrocarbon or water from wells penetrating undergroundformations. The effective removal of damage can also increase theinjectivity of injection wells.

The production rate of an oil, gas or water well following drilling andworkover operations is often limited due to the presence of filter cakesgenerated during the operations. The filter cake must be removed inorder to maximise production. In high permeability formations, there maybe sufficient draw down from fluid production to lift off the filtercake and restore formation permeability. Generally however, it isnecessary to apply a chemical treatment to remove the filter cake.Conventional treatments for removing filtercake include the use ofaqueous solutions of an oxidiser, hydrochloric acid solutions, organicacid solutions including formic or acetic acid, combinations of acid andoxidiser, and aqueous solutions of enzymes which are introduced into theformation after the drilling or workover.

U.S. Pat. No. 6,140,277 and PCT/GB00/01032 detail the problems whichaffect conventional techniques of filtercake removal. The effectiveplacement of reactive fluids such as hydrochloric acid is very difficultand generally results in very variable effectiveness of treatment alongthe wellbore or other target zone. Placement problems have beenaddressed by methods which generate acid in-situ (U.S. Pat. No.5,678,632; PCT/GB00/01032).

Another problem is that filter cakes frequently consist of severalcomponents which are generally not treatable by a single treatment. Forexample, certain drilling muds or drill-in fluids contain calciumcarbonate or dolomite in combination with a polymer or polymers whichprovides suitable Theological properties. Both the carbonate and thepolymer contribute to the formation of a filter cake. Rock finesgenerated during drilling of the formation rocks may also be present inthe filter cakes.

An acid may be used to dissolve the carbonate components of filter cakesand suitable breakers such as oxidizing agents or enzymes may be used tobreak down polymers in the filter cake. These have generally beenapplied as separate treatments as acids and polymer breakers arefrequently incompatible (U.S. Pat. No. 6,140,277; PCT/GB00/01032).

U.S. Pat. No. 6,140,277 teaches the use of formulations comprising aviscoelastic surfactant, a chelating agent and an enzyme for breaking afilter cake.

PCT/GB00/01032 teaches the use of formulations comprising esters whichhydrolyse (optionally using ester hydrolysing enzymes) to produceorganic acid in-situ in combination with oxidants or enzyme polymerbreakers to treat formation damage including filter cakes.

Both U.S. Pat. No. 6,140,277 and PCT/GB00/01032 allow single stagetreatments of filter cakes wherein the treatment dissolves carbonate andhydrolyses polymers.

In addition to treatment fluid placement problems and the problems oftreating more than one component in filter cakes, additional problemsarise in removing filter cakes during sand control completions.

Sand control is often required when producing oil gas or water fromsandstone formations. A large proportion of hydrocarbon productionworldwide is from underground sandstone formations. These formationsoften have a high porosity and permeability so have the potential toproduce hydrocarbons at high rates. Frequently however, such formationshave a tendency to produce sand, due to being unconsolidated or poorlyconsolidated. Sand as used herein refers to fine particulate materialswhich may be produced from poorly consolidated sandstones. Normallythese will be sand grains. Poorly consolidated sandstones have beensuitably defined in U.S. Pat. No. 3,741,308. Sand producing formationsgenerally are relatively young in the sense of geological time and aresimply composed of loosely attached sand or sediments that have not yetbeen converted to solid sandstone by geochemical methods.

Factors which cause sand production in weak formations include producingdrawdown, pressure depletion, in-situ rock stresses, changes in flowrate or changes in water cut (sand production is often associated withwater breakthrough). While a certain amount of sand production can betolerated, excessive sand production can cause a variety of operationalproblems including erosion of pumps, tubing, chokes, valves and pipebends. This can lead to serious safety and environmental consequences(U.S. Pat. No. 3,741,308). It can also lead to collapse of the formationor casing and significant reduction in or loss of production.

It is therefore normal practice to seek to put sand control measures inplace in wells drilled into formations which are expected to producesand. The tendency of the formation to produce sand is indicated by theunconfined compressive strength of the formation. Generally if aformation has an unconfined compressive strength of about 1,100 p.s.i.or greater, sand production is unlikely so sand control measures are notlikely to be required. At an unconfined compressive strength of betweenabout 400 & 1,100 p.s.i. sand production may occur and sand control isnormally desirable. Below an unconfined compressive strength of about400 p.s.i. sand control is almost certainly required.

There are a number of established approaches to sand control. Theseinclude mechanical approaches which physically prevent sand fromentering the produced fluids and the use of chemical methods which bindthe sand grains together.

Common approaches to mechanical sand control are gravel packing and theuse of pre-packed screens. Gravel packs use gravel (sized sand) placedin the wallbore and physically prevent sand from entering the productionstream. A screen is used to prevent gravel production. Gravel packs maybe open hole (external gravel pack) or cased hole (internal gravelpack). “Frac-packs” combine cased hole gravel pack and hydraulicfracturing completions and are generally expected to give higherproductivity than straight gravel packing. Pre-packed screens arecommonly used in horizontal openhole wells and typically consist of alayer of resin bonded gravel held between two screens.

The process of placing the gravel in internal and external gravel packsinvolves pumping a slurry of gravel suspended in a carrier fluid. Tomaintain circulation of this fluid and effective gravel placement forexternal gravel packs, particularly in long horizontal or deviatedwells, drilling mud filter cake on the face of the wallbore plays animportant role in preventing fluid loss and maintaining circulation. Inthe case of external gravel packs, it is therefore very important thatthe filter cake remains intact during placement of the gravel. An intactfilter cake is also desirable when placing pre-packed screens, toprevent fluid loss. Avoiding fluid loss is extremely important in thecase of long horizontal or deviated wells where the producing intervalmay be several thousand feet long.

In sand control completions, the entrapment of filter cake between theformation and screens or gravel can potentially result in a significantproductivity reduction. Once gravel packing has been completed, or afterpre-packed screens have been placed in the wellbore, effective removalof the filter cake is necessary to maximise production or injectionrates. Society of Petroleum Engineers paper SPE 50673 describes thestate of the art with respect to clean up of sand control completions inopen hole horizontal wells.

U.S. Pat. No. 6,140,277 teaches that it would be highly advantageous ifthe fluid used to deliver the gravel could also be used to dissolve thefilter cake. This would eliminate the need for a separate treatment justto dissolve the filter cake and result in substantial time and costsavings. The fluid therefore needs two contradictory attributes: thefilter cake must not be degraded prematurely (before placement of thegravel pack is completed) but the fluid should eventually dissolve thefilter cake.

It is not possible to add acid directly to the gravel packing fluid asthis would break a filter cake very quickly leading to premature fluidleak off which would adversely affect both the gravel packing operationand the efficiency of filter cake treatment along the remainder of thewellbore.

U.S. Pat. No. 6,140,277 teaches that there is an urgent need in thedrilling and completions sector for a reliable fluid for degradingfiltercake quickly, efficiently and completely and which can be used asa carrier fluid in conjunction with other completion, workover orstimulation operations. U.S. Pat. No. 6,140,277 further teaches the useof a formulation containing a viscoelastic surfactant, chelating agentand enzyme to place gravel and dissolve filter cake.

In addition to the effective clean up of filter cakes in openhole gravelpacks, it is also desirable to keep the screens used in sand controlcompletions as clean as possible during placement in the undergroundformation and during subsequent well construction operations. This isdifficult in inclined, deviated or horizontal wellbores as the equipmentmay lie against the wall of the wellbore and drilling mud, formationfines and other undesirable materials may be forced into the screen.Effective clean up of the screens may be very difficult. Other downholeproduction equipment may also be damaged by particulate material whichenters regions of the equipment which should be kept clean. Use ofcentralisers or functionally equivalent means of preventing theequipment contacting the wellbore may assist in keeping the equipmentclean.

The object of the present invention is to provide an alternative processfor disrupting, for instance degrading, a filter cake present in anunderground formation.

A further object of the present invention is to provide a process fordegrading a filter cake in gravel packing and other sand controloperations.

Another object of the present invention is to provide a process ofdegrading a filter cake in which the rate of degradation of the filtercake is controlled and is highly predictable.

It is an additional object of the present invention to provide a processof degrading a filter cake which is environmentally acceptable byutilising components which are of low environmental impact.

Another object of the present invention is to provide means ofpreventing damage to screens and other downhole equipment duringplacement in the underground formation.

Accordingly, the present invention provides a process for disruptingfilter cake in an underground formation, which process comprises:

-   -   a) incorporating into a treatment fluid a solid polymer capable        of being converted by hydrolysis into one or more organic acids;    -   b) introducing the treatment fluid into the underground        formation; and    -   c) allowing the solid polymer to hydrolyse in the presence of        water to produce organic acid such that acid soluble material        within the filter cake or adjacent formation is dissolved.

The invention also provides the use of a solid polymer, which is capableof being converted by hydrolysis into one or more organic acids, as afilter cake disrupting agent in an underground formation treatmentfluid.

The process of the present invention may be used to treat formationdamage present within the wellbore or adjacent parts of the undergroundformation. In particular it may be used to treat, and thereby disrupt,degrade or remove altogether, filter cakes which arise from drilling orworkover operations.

A well is drilled to the underground formation to be treated. The wellis generally an openhole completion wherein the inside of the wellboreis lined with a filter cake. The filter cake normally needs to be atleast partly removed to allow the well to produce at high rates. Casingand perforating of a well will generally bypass formation damage causedby filter cakes so the process of the present invention is not normallyapplicable to such wells, unless there is formation damage amenable totreatment using formulations of the present invention.

The well bore serves as a convenient means for introducing the treatmentfluid into the formation by any method known to those skilled in the artincluding via the drillstring (in which case the mud pumps may be used),coiled tubing or bullheading of the fluid.

The polymer used in the process of the present invention is any solidpolymer which hydrolyses in the presence of water to generate an organicacid or acids. Preferably the polymer is a polyester, most preferably analiphatic polyester selected from the group which can be synthesised bysuitable processes known to those skilled in the art, including the ringopening melt condensation of lactide (lactic acid cyclic dimer),glycolide (glycolic acid cyclic dimer) and caprolactone. Suitablepolymers include polylactide (polylactic acid) polyglycolide(polyglycolic acid) lactide-glycolide copolymer, lactide-caprolactonecopolymer, glycolide-caprolactone copolymer orlactide-glycolide-caprolactone copolymer.

Hydrolysis of a polymer produced from the condensation of lactideproduces lactic acid and hydrolysis of a polymer produced from thecondensation of glycolide produces glycolic acid. Lactic acid andglycolic acid (hydroxyacetic) acid are the preferred acids produced byhydrolysis of the polymer used in the process of the present invention.Suitable polymers also include homopolymers or copolymers of lactic acidand hydroxyacetic acid (glycolic acid) and copolymers of lactic acidand/or glycolic acid with one or more other compounds containinghydroxy-, carboxylic- or hydroxycarboxylic acid moieties. U.S. Pat. No.4,986,353 provides examples of suitable monomers with which lactic acidor glycolic acid may be condensed. Suitable monomers include but are notlimited to tribasic acids such as citric acid, dibasic acids such asadipic acid, and diols such as ethylene glycol and polyols. They alsoinclude difunctional molecules such as 2,2-(bishydroxymethyl) propanoicacid. Preferred co-condensing molecules according to the process of U.S.Pat. No. 4,986,353 are citric acid, 2,2-(bishydroxymethyl) propanoicacid, trimethylol-ethane, and adipic acid. These, or any other monomersmay also be incorporated into the polymers according to the process ofthe present invention as long as the solid polymer undergoes hydrolysisin the presence of water to generate an organic acid or acids.

Acid production is from simple hydrolysis of ester linkages in thepolyester.

Polymers which hydrolyse to produce lactic acid and/or glycolic acid arepreferred. The most preferred polymers are aliphatic polyesters selectedfrom the group which can be synthesised by the condensation of lacticacid, glycolic acid and caprolactone. The composition of the polymer orcopolymer is a principal determinant of the hydrolysis rate of thepolymer. A composition that will give the required rate of hydrolysisunder the temperature conditions of the treated formation will generallybe selected. After placement of the polymer, the well will normally beshut in for a time sufficient for the polymer to hydrolyse and produceacid and dissolve acid soluble material.

Preferably, the organic acids produced by the hydrolysis of the polymerreact with calcium carbonate to form calcium salts with a solubility inwater of at least a few percent at the formation temperature. Lacticacid and glycolic acid are suitable acids.

The type of organic acid, amount of acid delivered and rate of acidproduction at a given temperature may be determined by selecting anappropriate polymer composition and form of presentation of the solidpolymer (size and shape of the solids) and the quantity of polymer inthe treatment fluid.

Hydrolysis of the polymer is by bulk erosion (Biodegradable Polymers asDrug Delivery Systems, Edited by Mark Chasin and Robert Langer. MarcelDekker Inc., New York, Basel and Hong Kong, 1990). The rate ofhydrolysis is primarily influenced by four key variables; monomerstereochemistry (D or L form), comonomer ratio, polymer chain linearityand polymer molecular weight. As hydrolysis takes place at the surfaceof the polymer, for a given polymer composition, the particle size ofthe polymer is also a prime determinant of the rate of hydrolysis andacid production. Smaller particles of a polymer of a given compositionat a given temperature have a larger surface area per unit weight thanlarger particles so will produce acid at a faster rate. In general,polylactic acid and other lactic acid rich polymers will degrade at aslower rate than polyglycolic acid and glycolic acid rich polymers.Incorporation of caprolactone into the polymers can further increase therate of hydrolysis of the polymers. The rate of hydrolysis of thepolymers may also be influenced by the extent of block or randomstructure in copolymers, by chemical modification of the end groups ofthe polymer or by the introduction of branching into the polymers, forexample by incorporating polyols into the polymer.

The rate of depolymerisation may also be increased by incorporatingspecific chemicals such as quaternary ammonium compounds into thepolyesters (U.S. Pat. No. 5,278,256). Compositions of polymer whichhydrolyse relatively quickly include polyglycolic acid and polymerswhere glycolic acid represents over 50% of the constituent monomers ofthe polymer. Compositions of polymer which hydrolyse relatively slowlyinclude polylactic acid and polymers where lactic acid represents over50% of the constituent monomers of the polymer. In low temperatures, forexample from 20° C. to 100° C., polymers rich in glycolic acid (i.e.containing over 50%) will tend to be used in the process of the presentinvention. At higher temperatures, for example from about 80° to 170° C.polymers rich in lactic acid (i.e. containing over 50%) will tend to beused.

Sufficient polymer is present in the treatment fluid to producesufficent acid, when the polymer is hydrolysed, to have a substantiveeffect on filter cake. By substantive effect it is meant that sufficientacid is produced on the hydrolysis of the polymer to give sufficientdissolution of acid soluble material, present in or adjacent to thefilter cake, to assist in disruption of the filter cake and/or theremoval of damage in the underground formation. The process of theinvention thereby serves to increase the permeability of the undergroundformation. Typically the permeability of the underground formation issubstantially restored to, and may even exceed, the level it would havebeen without the formation of filter cake.

Disruption of filter cake in accordance with the process of theinvention may be achieved, for example, by the dissolution ofacid-soluble material, such as carbonate, present in the filter cake.Alternatively, or in addition, disruption of filter cake in the processof the invention may be achieved by the dissolution of acid-solublematerial, for instance carbonate rock, adjacent to the filter cake. Toobtain sufficient dissolution, normally several percent w/v, at leastabout 1 to 2% and preferably about 2 to 10% w/v of polymer isincorporated into the treatment fluid. Higher amounts may be used if itis determined that this is beneficial.

The polymer may be used in underground formations at any temperature upto at least the melting temperature of the selected polymer. Forexample, poly(L-)lactic acid has a melting temperature of about 173° C.and polyglycolic acid has a melting temperature of 230° C. The processmay however be usefully operated at temperatures of as low as 20° C. Informations at or above the melting temperature of the selected polymer,pre-cooling of the formation by injection of a large volume of waterahead of the treatment fluid containing the polymer may optionally beemployed. The cooling effects of any preflush and of the treatment fluidcontaining the polymer, will be taken into account in calculating therequired shut-in period. Because acid is produced over a period of time,the solid material may be placed within the formation before most of theacid is produced. Acid is then delivered to the whole zone in whichcontact with the polymer occurs.

The polymers may be used in any solid configuration, including, but notbeing limited to spheres, cylinders, cuboids, fibres, powders, beads orany other configuration which can be introduced into the formation. Itwill preferably be used in the form of particles in the size range 1micron to 2 mm, most preferably 10 microns to 1 mm.

Polymers of the desired size and shape may be prepared by any suitableprocess known to those skilled in the art including but not beinglimited to high sheer dispersion of the polymer melt, emulsificationfollowed by solvent evaporation, desolvation, spray drying or grinding.Some suitable processes of producing microparticles, microspheres,microcapsules, shaped particles and fibres are reviewed in Chasin, M andLanger, R. (Eds.). Biodegradable Polymers as Drug Delivery Systems.Marcel Dekker Inc., New York, (1990). U.S. Pat. No. 4,986,355 teaches aprocess of preparing suitably sized polyester particles for use as afluid loss additive or as a gel breaker in a subterranean formation.

In general it is desirable to avoid the use of chlorinated solvents insolvent based methods of producing the particles of the desired size andshape. For example methylene chloride has been used to producemicro-particles of polyesters such as polylactide for use in drugdelivery applications, but significant amounts of methylene chloride maybe present in the micro-particles even after drying. The presence ofchlorinated solvents will reduce the otherwise excellent environmentalacceptability of the polyesters. The solubility of polyesters innon-chlorinated solvents is generally limited.

The polymer particles of the present invention are introduced into theformation as a slurry or suspension with or without a suspending agentor a viscosifying agent such as borate crosslinked guar gum or any othersuitable viscosifying agent. If viscosifying the treatment fluid, use ofgel systems such as guar-borate which are “broken” (i.e. have theirviscosity reduced) by acid produced from hydrolysis of the polymer arepreferred, although specific gel breakers such as oxidants or enzymesmay also be incorporated into the treatment fluid containing thepolymer.

As discussed above, the rate of hydrolysis of the solid polymer may becontrolled by modifying its chemical composition and/or its physicalsize and shape. The pH and the presence of catalysts may also affect therate of hydrolysis.

In some embodiments of the invention the solid polymer may be used as acoating for other particles placed in the underground formation, such asgravel used in gravel packing, or may be contained within otherparticles placed in the underground formation, such as porous proppantmaterials. In these cases, acid will still be produced by the hydrolysisof the solid polymer and dissolve acid soluble materials within theunderground formation. Methods of coating particles with the solidpolymer or of incorporating the solid polymer into other particles willbe known to those skilled in the art.

In other embodiments of the invention the solid polymer may be used as acoating for equipment placed in the underground formation. The equipmentmay include screens, such as gravel pack screens and prepacked screensused for sand control, or other downhole production equipment. Whereused as a coating for equipment the solid polymer, while stillcontributing to acid production in the underground formation will alsoprotect the surface of the equipment during placement in the formationand preventingress of drilling mud, formation fines and otherundesirable materials into the screen, prepacked screen or otherequipment.

The solid polymer may also be present within at least part of theinternal spaces of the screens or production equipment. Hydrolysis ofthe solid polymer will again contribute to the production of acid in theunderground formation but as in the case of a coating, willpreventingress of drilling mud, formation fines and other undesirablematerials into the screen, prepacked screen or other equipment.

If desired, the solid polymer may be present as both a coating and inthe internal spaces of the equipment. After placement of the equipmentin the underground formation the eventual complete dissolution of thesolid polymer will result in the underground equipment being availablein a clean state.

To assist in prevention of damage during placement of equipment in theunderground formation the solid polymer may also be used in a mouldedform as a centraliser to keep the equipment from contact with the sideof the wellbore. For example screens may be kept in the centre of thewellbore during placement in an openhole horizontal wellbore prior togravel packing. The eventual complete dissolution of the solid polymerwill contribute to the production of acid in the underground formation.

Coating of equipment with solid polymer, placement of solid polymerwithin equipment and moulding of centralisers out of solid polymer maybe by any method known to those skilled in the art.

Where acid production alone is enough to sufficiently dissolve acidsoluble materials in the filter cake and increase formationpermeability, use of a treatment fluid containing only acid producingpolymer will be used. In most cases however, filter cakes will alsocontain polymers added to the drilling fluid as fluid loss additives andviscosifying polymers so polymer breakers will also be incorporated intothe treatment fluid. These will also be introduced into the undergroundformation, where they serve to degrade polymeric material (such asviscosifying polymers) present within filter cakes in the formulation.

Preferred polymer breakers of the present invention are oxidativebreakers (oxidants) and enzyme breakers, although any other breakercapable of at least partly degrading viscosifying polymers may also beused. Polymer breakers will generally be used at least thatconcentration known to be effective by those skilled in the art.Sufficient polymer breaker is present in the treatment fluid to have asubstantive effect on filter cakes containing those polymers which canbe broken by the polymer breakers. By substantive effect it is meantthat sufficient polymer is hydrolysed to assist in the disruption of thefilter cake and the removal of near wellbore damage attributable to thepresence of polymers.

Oxidative breakers used in the process of the present invention may beany one of those oxidative breakers known in the art to be useful toreact with viscosifying polymers, in most cases polysaccharides, toreduce the viscosity of viscosifying polymer containing compositions orto disrupt viscosifying polymer containing filter cakes. The oxidativebreaker may be present in solution or as a dispersion. Suitablecompounds include peroxides, persulphates, perborates, percarbonates,perphosphates, hypochlorites, persilicates and hydrogen peroxide adductssuch as urea hydrogen peroxide and magnesium peroxide.

Preferred oxidative breakers for incorporation into treatment fluids tobe used in the present invention are peroxides which can decompose togenerate hydrogen peroxide.

Of the oxidative breakers most preferred are percarbonates andperborates, most especially sodium percarbonate and sodium perborate.

Preferred enzyme breakers for use in the process of the presentinvention include those enzymes known in the art to be useful tohydrolyse viscosifying polymers and thereby to reduce the viscosity ofviscosifying polymer containing compositions or of viscosifying polymercontaining filter cakes. Enzyme breakers will be selected on the basisof their known ability to hydrolyse the viscosifying polymer. Normallythe viscosifying polymer will be a polysaccharide and the enzymebreakers will be selected on the basis of their known ability tohydrolyse the polysaccharide components in the filter cake. Examples ofsuitable enzymes which may be used to break polysaccharides includeenzymes which can hydrolyse starch, xanthan, cellulose, guar,scleroglucan, succinoglycan or derivatives of these polymers.

In some embodiments of the present invention the effectiveness of theincorporated oxidant breakers can be enhanced by producing more reactiveoxidants. Under certain conditions, for instance when a peroxide isincluded in the treatment fluid, the production of hydrogen peroxide inthe presence of the organic acid may result in the formation of aperacid which is a more effective oxidant than the hydrogen peroxide.

Hydrolysis of esters in the presence of hydrogen peroxide may alsoresult in the production of peracids. Esters are known to be hydrolysedby hydrolases (EC 3) such as a lipase (EC 3.1.1.3), an esterase (EC3.1.1.1) or a protease (EC 3.4) in the presence of hydrogen peroxide orother peroxides to form a peracid (U.S. Pat. No. 3,974,082; U.S. Pat.No. 5,108,457; U.S. Pat. No. 5,296,161; U.S. Pat. No. 5,338,474; U.S.Pat. No. 5,352,594; U.S. Pat. No. 5,364,554). Peracids produced in-situby such enzymes have been used for bleaching applications. Peracids aremore effective oxidants than peroxides, particularly in the temperaturerange 25 to 80° C. Accordingly, esters, ester hydrolyzing enzymes,hydrogen peroxide or hydrogen peroxide generating compounds may beincorporated into treatment fluids of the present invention. Hydrolysisof polyesters in the presence of hydrogen peroxide is also expected togenerate peracids.

In some embodiments of the present invention, it may be desirable toincorporate more than one type of polymer breaker, for example anoxidant might be used in combination with an enzyme breaker in the casewhere two polysaccharides are present but only one is amenable to attackby an enzyme. Oxidants and enzymes may if desired also be used in theform of delayed release preparations, such as will be well known bythose skilled in the art.

The solid polymer particles are introduced into the formation as aslurry or suspension with or without a viscosifying agent such as boratecrosslinked guar gum or any other viscosifying agent. The use of gelsystems such as guar-borate which are “broken” (i.e. have theirviscosity reduced) by acid produced from hydrolysis of the polymer ispreferred, although specific gel breakers such as oxidants or enzymesmay also be incorporated into the treatment fluid containing the polymerand may act on other types of gels which are not broken by pH reduction.

Additional materials including chemicals, catalysts or enzymes may beincorporated into the treatment fluid by dissolution or dispersion. Suchmaterials may additionally or instead be incorporated into the solidpolymer by dissolution, dispersion or encapsulation by any method knownto those skilled in the art.

The additional materials may have functional activity or activities asoilfield chemicals, including production chemicals. Examples of suchfunctional activities include, but not limited to, activity as a gel orpolymer breaker, acid, corrosion inhibitor, surfactants, scaleinhibitors, chelating agent, scale dissolvers, pour point modifiers,paraffin inhibitors, asphaltene inhibitors, solvents, catalysts orbioactive agents, which may be used in the process of the presentinvention to assist in disruption of the filter cake or to addressproblems associated with hydrocarbon or water production.

In one embodiment of the process of the invention as defined above, atleast a portion of the polymer remains in the underground formation andcontinuously releases organic acid and a production chemical duringhydrocarbon production or water injection until the polymer hascompletely hydrolysed.

One function of the added materials is to adjust the specific gravity ofthe treatment fluid and solid polymer to the desired value for placementin the formation. Preferred materials for adjusting the specific gravityinclude water soluble alkali metal salts and other salts used foradjusting the specific gravity of oilfield brines.

Where solid polymers contain other materials by dissolution, dispersionor encapsulation, hydrolysis of the solid polymer will release the othermaterials. In the case of materials encapsulated in the polymer, releasewill generally follow acid production and in the case of dissolved ordispersed materials, release will be coincident with acid production.

Because acid is produced by the hydrolysis of the solid polymers,incorporation into the treatment fluid of chemicals which react withacid to produce desirable oxidants or other chemicals for treatment ofthe underground formation is convenient. Such acid reactive chemicalsmay be incorporated into either the treatment fluid, the solid polymercomponent of the fluid, or both. Examples of suitable chemicals arecalcium peroxide and ammonium bifluoride. Calcium peroxide decomposes inthe presence of acid to form hydrogen peroxide and ammonium bifluoridedecomposes in the presence of acid to form hydrogen fluoride. Productionof hydrogen fluoride permits the dissolution of materials which are notreadily soluble in organic acids solutions.

More than one polymer with or without encapsulated, dissolved ordispersed other materials, chemicals, catalysts or enzymes may beintroduced into the formation at the same time. For example, a fastdissolving polymer may be selected to give relatively rapid acidproduction. This may be used in combination with another slow dissolvingpolymer containing a well treatment chemical such as a scale inhibitorto give controlled release of the well treatment chemical duringsubsequent production operations. The eventual complete dissolution ofthe solid polymers allows ideal clean up behavior.

All chemicals required for the process of the present invention willnormally be technical grade to reduce the cost of the process.

Where an enzyme is used as a polymer breaker according to the process ofthe present invention, it is necessary to select an enzyme which remainsactive under reservoir conditions and in the treatment fluid for atleast as long as the catalytic activity is needed.

The enzyme is generally a water soluble enzyme. It is generallyadvantageous for the enzymes to be readily water soluble although theenzymes may also be active and be used in low water activityenvironments or two-phase systems such as emulsions or dispersions.Typically, isolated enzymes are used. Enzymes may be isolated fromplant, animal, bacterial or fungal sources. The enzymes may be producedfrom wild-type, conventionally bred, mutated or genetically engineeredorganisms. The enzymes may, optionally, be chemically modified, as longas they retain or possess the desired catalytic ability. Preferably, theenzymes will be industrial enzymes available in bulk from commercialsources.

Where it is desired to treat filter cake during gravel packingoperations the solid polymer and optionally polymer breakers of thepresent invention will be incorporated into the carrier fluid for thegravel pack.

Where desired, the viscosity of the solution will be adjusted to thechosen value using viscosifying polymers or viscosifying surfactantswith the characteristics required for gravel packing operations.

Suitable sizes and ratios of gravel and solid polymer will be selectedto give the desired packing in the gravel pack and to produce thedesired amount of acid. The solid polymer may be used at anyconcentration which will result in subsequent removal of as least aportion of the acid soluble material in the filter cake. Suitable sizesand ratios will be known to or may be readily determined by thoseskilled in the art.

The gravel packing fluid, containing gravel and solid polymer andoptionally polymer breakers may be prepared and placed in the formationby any method of gravel packing such as is well known to those skilledin the art.

The polymer will become distributed throughout the gravel pack and acidwill be delivered to the whole volume of the gravel pack, including thatportion in the immediate proximity of the filter cake lining thewellbore. Polymer breakers, if optionally incorporated will also bedelivered to the whole volume of the gravel pack.

Although most of the acid is produced from hydrolysis of the solidpolymer after placement of the solid polymer in the undergroundformation, traces of acid present in the treatment fluid couldpotentially lead to premature dissolution of the filtercake. This couldprove to be a particular problem in situations where large volumes offluid may be in contact with relatively small areas of filter cake, forexample during the gravel packing of long horizontal wells where a largevolume of even a very dilute acid could lead to premature erosion of aportion of the filter cake leading to fluid leak off, lost circulationand failure of the gravel pack. This situation can be avoided byincorporation into the treatment fluids and gravel packing fluidsaccording to the present invention of a suitable amount of a buffer,such as for example an alkaline borate buffer, which would maintain thepH at a level above that at which erosion of the filter cake would occurfor at least the period of time required to complete the gravel pack.

Similarly, premature dissolution of filter cake due to polymer breakersattacking the filter cake may be addressed by using controlled releasepreparations of polymer breaker such as will be known to those skilledin the art, including, but not being limited to, preparations whereinthe polymer breaker is incorporated into the solid polymer byencapsulation, dissolution or dispersion.

The treatment fluid is normally prepared by dissolving or dispersing thesolid polymer and polymer breaker in suitable water for example city(drinking) water, produced water or sea water. If it is preferred, thetreatment fluid may be prepared by adding the individual components towater on a continuous, preferably carefully controlled and monitoredbasis as the fluid is injected into the underground reservoir. Othermethods of preparing the treatment fluid will be well known to thoseskilled in the art. A single solution or dispersion containing all ofthe components is preferably used.

The concentrations of solid polymer and polymer breakers present in thetreatment fluid will depend on the amounts of acid and breaker requiredto disrupt the filter cake. Typically, sufficient solid polymer toproduce between 0.5% and 10% w/v organic acid when fully hydrolysed willbe used. Enzyme polymer breakers are typically used at 0.05% to 5% v/vof liquid preparations or the equivalent amount of dry enzymepreparation. Amounts of oxidative breaker used will depend on the typeof breaker employed but will be of the order of 0.005 to 60 Kg/m3,preferably 0.2 to 10 Kg/m3.

For near wellbore treatments, the volume of treatment fluid introducedinto the formation will typically be at least equal to the wellborevolume plus an allowance for some leak off into the formation. A fluidvolume of between 120% and 200% of the wellbore volume will normally beused although if a high rate of fluid loss is expected a volume up to300% or higher of the well bore volume may be selected. For gravelpacking and frac-pack treatments a volume of fluid appropriate to theneeds of the treatment will be used and will be readily determined bythose skilled in the art. The solid polymer will normally be of a sizethat will not penetrate far into the formation so will be retainedwithin, and produce acid within, the wellbore, gravel pack, frac-pack orfracture.

The treatment fluid needs to be shut in for a period long enough for thedesired amount of acid to be produced by hydrolysis of the solidpolymer, for the acid produced to dissolve acid soluble materials, andfor any polymer breakers present to break the polymers. Normally it isdesirable for the treatment to be completed within 1 to 3 days. In somecircumstances, it may be acceptable for the treatment to take longer.For example, some wells are drilled and completed but then shut in foran extended time (of at least several weeks) before being put onproduction. The treatment fluid may therefore be left in the wellbore toremove filter cake during the extended shut in. The well will normallybe shut in after introduction of the treatment fluid for a period,typically between 2 hours and a week, preferably 6-48 hours, to allowproduction of acid and breaking of the polymer. The well is then put onor returned to production, or in the case of injection wells, put oninjection.

The treatment fluid may contain further materials or chemical additivessuch as are commonly used in the oil industry if their inclusion isdeemed to be beneficial and if they are compatible with the othercomponents of the treatment fluid.

Generally the treatment fluid will be aqueous, although in very hightemperature formations, a suitable hydrocarbon or a mutual solvent maybe used to reduce the rate of hydrolysis of the solid polymer.

The present invention has the following particular advantages over theprior art:

The process provides a simple, effective and convenient way to treatfilter cake containing both carbonate and polymers using a single fluid.

Also the process is generally a very low hazard process compared toprevious methods involving a substantive degree of acidising.

The components of the system are generally environmentally acceptable.The polyesters, enzymes and certain oxidant components such as thepercarbonates are of low environmental impact. Also, the fluids are nothighly corrosive, meaning that the use of corrosion inhibitors isgenerally not required which gives additional environmental benefits.

The invention will be further illustrated by the following examples:

EXAMPLE 1

1 g of polyglycolic acid powder was added to tubes containing 10 ml ofwater and 2 g of calcium carbonate (average particle size 50 microns).The tubes were capped and incubated at 25° C., 60° C. and 80° C. Calciumcarbonate dissolution (due to glycolic acid liberated by hydrolysis ofthe polyglycolic acid) was monitored by taking samples of the aqueousfluid, separating particulate material by centrifugation and analyzingthe soluble calcium using a colorimetric assay method. The amount ofcalcium carbonate dissolved after 24 hours was 6, 20 and 40 g/l at 25°C., 60° C. and 80° C. respectively.

EXAMPLE 2

1 g of polylactic acid granules (average 2.5 mm diameter) was added totubes containing 10 ml of water and 2 g of calcium carbonate (averageparticle size 50 microns). The tubes were capped and incubated at 80° C.and 95° C. Calcium carbonate dissolution (due to lactic acid liberatedby hydrolysis of the polylactic acid) was monitored by taking samples ofthe aqueous fluid, separating particulate material by centrifugation andanalyzing the soluble calcium using a colorimetric assay process.

The amount of calcium carbonate dissolved after 24 hours was 2.5 and12.4 g/l at 80° C. and 95° C. respectively.

EXAMPLE 3

1.000 g of ground polylactic acid (average diameter 400 microns) wasadded to 75 ml of deionised water at placed in an Ofite high pressurehigh temperature (HPHT) cell. The cell was sealed and heated to 121degrees C. After approximately 23 hours, 48 hours or 69 hours (threeseparate runs) the cell was opened and any undissolved materialcollected and air-dried to constant weight. The percentage of theinitial PLA dissolved was then determined.

Time at Percent 121° C. PLA pH of solution (by (hours) dissolved pHindicator strip) 0 0 23 60 3 48 98  2* 69 100 2 (*2.85 by pH meter)These results indicate that hydrolysis requires tens of hours at 121degrees C. The low pH indicates that production of lactic acid isassociated with the hydrolysis of the polylactic acid.

The Examples show that calcium carbonate is dissolved by acid producedfrom the hydrolysis of the solid polymers. They further show that therate of acid production is a function of the composition of the polymerand the temperature. Organic acid production from polymers introducedinto an underground formation will lead to disruption of carbonatecontaining filter cakes or other types of filter cake in contact with acarbonate formation.

1-40. (canceled)
 41. A process for preventing damage to screens andother underground equipment during placement in an underground formationwhich comprises coating of the equipment with a solid polymer capable ofbeing converted by hydrolysis into one or more organic acids, whereinthe solid polymer is polylactide, lactide-glycolide copolymer,lactide-caprolactone copolymer, glycolide-caprolactone copolymer orlactide-glycolide-caprolactone copolymer.
 42. A process for preventingdamage to screens and other underground equipment during placement in anunderground formation which comprises incorporating into at least partof the internal spaces of the equipment a solid polymer that is capableof being converted by hydrolysis into one or more organic acids.
 43. Aprocess for preventing damage to screens and other underground equipmentduring placement in an underground formation which comprises using as acentraliser for the equipment a moulded form of a solid polymer which iscapable of being converted by hydrolysis into one or more organic acids.44. A screen or other underground equipment which is suitable for use inthe production of oil, gas or water from wells drilled into undergroundformations and which comprises, coated thereon and/or incorporated intoat least part of the internal spaces thereof, a solid polymer accordingto claim 41 which is capable of being converted by hydrolysis into oneor more acids.
 45. A screen or other underground equipment according toclaim 44 wherein the polymer is a polyester.
 46. A screen or otherunderground equipment according to claim 44 wherein the polymer ispolylactide, lactide-glycolide copolymer, lactide-caprolactonecopolymer, glycolide-caprolactone copolymer orlactide-glycolide-caprolactone copolymer.
 47. A screen or otherunderground equipment which is suitable for use in the production ofoil, gas or water from wells drilled into underground formations andwhich comprises, coated thereon and/or incorporated into at least partof the internal spaces thereof, a solid polymer according to claim 42which is capable of being converted by hydrolysis into one or moreacids.
 48. A screen or other underground equipment which is suitable foruse in the production of oil, gas or water from wells drilled intounderground formations and which comprises, coated thereon and/orincorporated into at least part of the internal spaces thereof, a solidpolymer according to claim 43 which is capable of being converted byhydrolysis into one or more acids.